In recent years and as a consequence of energy shortages throughout the world, much technical interest has been manifested in the development of sources of energy other than those relying on fossil fuels. Geothermal energy has for many years been viewed as a source of energy, in particular for electrical power generation, and has therefore in recent years been considered as an attractive alternate energy source. Geothermal energy does not result in significant production of gaseous pollutants and has attractive capital and operating cost benefits which are not enjoyed by conventional energy generation systems.
Geothermal energy sources typically rely on either a supply of steam or of superheated hot water found at varying depths below the surface of the earth. Sources of geothermal hot water are more common than geothermal steam and substantial effort has been made to extract heat energy from such sources and convert it to electrical power.
Typically this is accomplished by permitting the superheated water to flow out of a geothermal well, flashing it to form a steam phase, separating the available steam at the surface and driving a steam turbine with it and ultimately disposing of the remaining hot water.
Among the problems encountered in development of geothermal hot water wells for generation of electrical energy is the deposition of salts in the well and in the heat extraction and power generation equipment. It is not uncommon to have extremely high levels of dissolved solids in hot geothermal brines and as these brines are cooled in the flashing step and otherwise suffer dissipation of heat in the extraction procedure, these solids tend to deposit in the valves, pipes and other equipment. This causes fouling within the well and in surface pipe and equipment such as the steam separators. Typically, geothermal waters contain dissolved calcium, sodium and other minerals, and the salts which are deposited include sodium chloride, calcium and iron carbonates and heavy metal sulfides and silicates. The salt deposition necessitates either (1) frequent cleaning of the wellbore piping, and steam separators which is time consuming, expensive and disruptive of the operation; or (2) expensive chemical control by the introduction of additives to the fluid stream.
There are obvious thermodynamic advantages to be achieved by employing superheated brines at high temperatures. However, such brines may contain extraordinary amounts of dissolved salts and are referred to as hypersaline brines. Such brines may occur at temperatures up to 650.degree. F. and contain total dissolved solids in the amount of 100,000 to 300,000 ppm. Such brines are difficult to flow continuously in geothermal energy recovery systems because of the severe tendencies they manifest to deposit dissolved solids in the wellbore and in the downstream equipment. Upon steam flash, the saltier brines become supersaturated in sodium chloride. The deposition of that salt results in a thick, tough coating which can block and occlude pipes and foul valves within a matter of hours. A more common scale, but one which deposits less rapidly, contains silica in combination with varying amounts of a number of heavy metals, in particular, iron. Nonhypersaline (dilute) brines are typically rich in bicarbonate and deposit a calcium carbonate scale when they are flashed.
In some areas, for example the Imperial Valley of California, hypersaline brines and nonhypersaline brines are encountered at different depths of a geothermal well, the hypersaline brines being typically overlain with a nonhypersaline brine. In such instances each of the brines is hot enough to be considered as a geothermal source. However, producing either brine separately is problematic because each brine manifests its peculiar ability to deposit solids.